1. Field of the Invention
The present invention relates to a liquid crystal display panel and a liquid crystal display device. More specifically, the present invention relates to a liquid crystal display panel and a liquid crystal display device of fringe field switching (FFS) type.
2. Description of Related Art
In recent years, in substitution for a conventional cathode-ray tube, new display devices having a thin flat-type display panel using principles of liquid crystal, electroluminescence or the like have been frequently used. A liquid crystal display device which represents these new display devices has characteristics in that it can be driven with a low power voltage, in addition to its thinness and lightness. The liquid crystal display device includes a liquid crystal layer disposed between two substrates. One substrate is an array substrate forming a display area in which a plurality of pixels are arranged in matrix, and the other substrate is a color filter substrate.
In particular, in a thin film transistor (TFT)-type liquid crystal display device, a TFT which is a switching element is provided in each pixel on the array substrate, and each pixel is able to independently carry a voltage to drive a liquid crystal layer, thereby making it possible to achieve the display of high quality with little crosstalk. Each pixel includes a gate line (scan line) that controls ON/OFF of the TFT, and a source line (signal line) for inputting image data. Each pixel is typically formed in an area surrounded by the gate line and the source line.
An In-Plane switching (IPS)-type liquid crystal display device has one array substrate on which a plurality of pixel electrodes and common electrodes (opposed electrodes) are alternately arranged, so as to apply a substantially-horizontal electric field to the substrate surface for display. The IPS type liquid crystal display device has better view angle property compared with a typical Twisted Nematic (TN) type. However, the conventional IPS-type liquid crystal display device has a smaller light transmission rate compared with that of the typical TN type.
As a system in which this defect is improved, a fringe field switching (FFS) system has been suggested. The FFS-type liquid crystal display device is the system of achieving the display by applying a fringe electric field (an oblique electric field including both components of a horizontal electric field and a vertical electric field) to a liquid crystal layer. In the FFS-type liquid crystal display device, the pixel electrode and the common electrode are formed on one array substrate as is similar to the IPS system. However, the pixel electrode and the common electrode are overlapped with each other with an insulation film interposed therebetween. Typically, the lower electrode is a plate electrode (the lower electrode may be a plurality of branch-like electrodes). Furthermore the upper electrode includes a comb-tooth electrode including a plurality of branch electrode parts electrically connected in common, and gap parts therebetween.
Both of the comb-tooth electrode and the plate electrode are formed of transparent conductive films in the FFS system, thereby achieving particularly high light transmission rate. In the FFS system, a liquid crystal layer is driven by a fringe electric field between upper and lower electrodes, which makes it possible to drive a liquid crystal layer on a branch-like electrode part which is not a gap part of the comb-tooth electrode as well. The pixel electrode and the common electrode are formed of transparent conductive films, which can improve a light transmission rate compared with the IPS system in which light rarely transmits on the pixel electrode and the common electrode.
The FFS-type liquid crystal display device can be roughly divided into two structures. The first structure is that, on the array substrate, a common electrode which is a common potential is formed of a plate electrode arranged on the side of the lower layer, and a pixel electrode into which a pixel potential is written through a thin-film transistor is formed of a comb-tooth electrode arranged on a side of the upper layer (e.g., Japanese Unexamined Patent Application Publication Nos. 2002-31812 and 2005-234525). The second structure is that, on the array substrate, a common electrode and a pixel electrode are opposite to the arrangement stated above. Specifically, the common electrode is formed of a comb-tooth electrode in the upper layer, and the pixel electrode is formed of a plate electrode in the lower layer (e.g., Japanese Unexamined Patent Application Publication No. 2008-191669). Japanese Unexamined Patent Application Publication No. 2003-322869 discloses a structure in which one of the two electrodes of the plate electrode and the comb-tooth electrode is formed in the color filter substrate in place of the array substrate.
In the second structure stated above, a black matrix is typically arranged only in order to prevent color mixture of adjacent pixels. In other words, the black matrix having a width equal to or smaller than that of a source line is typically arranged on the source line. The reason that this structure can be employed is that it is possible to shield the electric field from the source line by arranging the comb-tooth electrode which is the common electrode on the source line while being overlapped with each other. In other words, this is because it is possible to prevent occurrence of a domain due to alignment disorder of liquid crystal due to an electric field from the source line. Further, Japanese Unexamined Patent Application Publication No. 2008-191669 described above discloses the structure that does not include the black matrix itself. By employing such a structure, even a part just beside the source line can contribute to the display, which can improve the light transmission rate.